Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
No, an alias will only give you pseudo-anonymity. Even trivial analysis like counting which words occur together frequently in your writings can reveal with very good accuracy any other alt of you, so the available information of you is basically everything you have shared online with enough accompanying self-written text.
Also, it’s not just about privacy, it’s about retaliation. It will be the easiest thing in the world for people to put together bots that will track the downvotes on every post they make and automate adding those people to block lists. Suddenly a whole fleet of alts is invisible to the people that would disagree with them.